As a radiation-exposure reduction measure in a nuclear plant that has been operated for several years to several decades, zinc is injected into a coolant of a nuclear reactor after the nuclear reactor has gone critical. For example, Patent document 1 discloses a technique of injecting zinc into a primary coolant in a state with an oxide film being formed on an inner surface of a piping of a primary cooling system of the nuclear reactor, which comes into contact with the primary coolant.